1. Field of the Invention
The present invention relates generally to pipe connector systems which utilize a bolt-ring for the field connection of pipes having a stub end, the bolt-ring being particularly adapted for the interconnection of polyolefin (HDPE) pipe.
2. Description of the Prior Art
There are a variety of pipe coupling and connector systems known in the prior art. Many of the prior art connectors were designed primarily for use in metal (iron or steel) piping systems. Typical applications included such areas as municipal water and sewage systems, chemical and petrochemical pipelines, and the like. Many of these piping systems employed solid metal flange coupling or connector rings. While these connectors worked satisfactorily in many instances, they were more expensive to produce because they were formed as solid metal rings. More recently, the so-called “convoluted” flange connectors have been introduced into the iron and steel pipeline industries. A convoluted flange utilizes a design in which an annular flange member has a U-shaped cross section to provide strength to the flange, which is reduced in weight and material content with resultant cost savings.
For example, U.S. Pat. No. 5,413,389 issued May 9, 1995 and entitled, “Cast Convoluted Piping Flange” describes a piping flange having a convoluted design with a transition in thickness from an outer rim to an inner rim of the flange so as to provide for stress management throughout the cross-sectional geometry. The convoluted design purports to provide the rigidity or stiffness necessary to insure a uniform coupling face for uniform deformation of a gasket or seal, while minimizing the weight of the flange by eliminating unnecessary material.
U.S. Pat. No. 4,458,924 issued Jul. 10, 1984, entitled, “Bimetal Flange Connector”, describes a bimetal flange that utilizes a hub of a first metal bonded to a rim of a second metal. The concept of a composite flange of two materials and the use of a recess to reduce weight and optimize stress distribution is described. Again, this connector was designed with a metal piping system in mind.
The convoluted flanges and flange couplers of the type described in the referenced patents have been used successfully in coupling metal pipes in many instances. Furthermore, the technique of computer aided stress analysis in the design of piping flanges has led to further improvements in the configuration of flanges that address the issue of stresses transmitted to the pipe, and in the design and construction of composite material flanges for metal piping systems.
There nevertheless exists a need for improvements in pipe coupling systems where the pipes are formed from thermoplastic materials such as polyethylene or another polyolefin. A popular thermoplastic piping material is high density polyethylene (HDPE). One commercially available system for HDPE piping systems is known in the industry as the “Van-Stone Style Polyethylene Pipe Joint.” While these systems have been used successfully for a number of years, the bolt-ring was a solid ring and thus had the disadvantages of weight and cost of manufacture discussed above with respect to the metal piping systems.
There are a number of reasons why a bolt-ring connector which is developed for use with a steel or ductile iron pipe might not be suitable for use with a thermoplastic material, such as polyethylene. Polyethylene is a visco-elastic material which naturally cold-flows under stress over time. The rate of strain is in proportion to stress intensity and time. When a solid steel bolt-ring is used on an HDPE flange and the HDPE thermally expands, the HDPE flange face cold-flows, i.e., is crushed by its own expansion against an immovable, fixed and excessively stiffbolt-ring. Upon cooling, the HDPE flange has typically cold-flowed to a reduced size flange thickness such that leaks can and do occur at the coupling. Upon freezing, the HDPE flange face thins by thermal contraction, the bolt-load is lost, the seal pressure diminishes, and leaks occur. When an HDPE flanged joint is bolted together using solid-metal coupling flange and very-stiff bolt-rings, cold flow of the HDPE stub-end usually occurs fairly quickly, typically within about eight hours. As a result, the contractor will often be forced to come back to the job site the next morning to re-tighten the coupling flange.
Additionally, HDPE exhibits expansion and contraction characteristics which are on the order of ten times those of steel under similar environmental conditions. Because the thermal expansion characteristics of steel are so much less than those of polyethylene, the prior art flange gaskets of the above type have been successfully employed in steel piping systems. HDPE, on the other hand, exhibits much greater expansion and contraction characteristics and also exhibits a “softness” which is about the same as the gasket materials which are used in the steel pipe coupling systems.
It would be advantageous, therefore, to provide an improved bolt-ring for connecting stub end thermoplastic pipe which could be elastically bent by sufficient bolt-load to accommodate the initial and long term cold flow of the polyolefin pipe material.
It would also be advantageous to provide such an improved bolt-ring for thermoplastic pipe which, in the presence of cold flow, would exhibit a residual bolt-ring deformation load sufficiently high to impose a positive pressure seal for the pipe coupling under all operating conditions.